If you're lower 2 thirds of the pack, stick to the T4. It is only marginally slower than the X4, but significantly more robust, and requires a lot less mechanical prep to keep close to its top running shape.

When I switched to the X4, I continued to have my son run the T4, even bought a 21 for him (the 20 is for sale BTW) - he's only a couple of tenths slower than me now on his fast laps, however he bangs boards much more often... The T4 will resist this treatment much longer to help him cross the finish line even after a rough run....